The Yellow-bellied Marmot has the thickset build characteristic of the genus. Its fur is relatively long and coarse, with buffy to yellowish hair running from the sides of the neck down along the chest. Because they favor herbaceous plants, their population density is often affected by the presence or absence of large grass-eating mammals. A moderate degree of grazing can increase the supply of the marmots' preferred herbs. Heavy grazing can reduce their food supply, if the grazing animals eat both grasses and herbs. Little or no grazing can also reduce the abundance of herbs because grasses out-compete herbs for space and soil nutrients. Yellow-bellied Marmots spend their summer days sunning (if the weather permits), grooming, and foraging. In hibernation, they depend for months on the fat stored in their bodies. Marmots that enter hibernation well-fattened have the best chance of surviving until spring.